1. Field of the Invention
The present invention relates generally to the field of integrated circuit fabrication, and more specifically to a method for repairing bump and divot defects in a phase shifting mask that does not require the classification of the defect as either a bump or divot defect.
2. Description of the Related Art
In the manufacture of semiconductor wafers, microlithography is used to pattern various layers on a wafer. A layer of resist is deposited on the wafer and exposed using an exposure tool and a template such as a mask or reticle (as used herein, "mask" shall refer to templates of any kind including masks and reticles). During the exposure process a form of radiant energy, such as ultraviolet light, is directed through the mask to selectively expose the resist in a desired pattern. The resist is then developed to remove either the exposed portions for a positive resist or the unexposed portions for a negative resist, thereby forming a resist mask on the wafer. The resist mask can then be used to protect underlying areas of the wafer during subsequent fabrication processes, such as deposition, etching, or ion implantation processes.
An individual reticle can cost up to $20,000 and typically requires up to two weeks to manufacture. Mask production likewise involves substantial time and expense. The complete circuit patterning for a modern IC will typically require 10 to 20 or more reticles, and the ability to repair a reticle or mask quickly saves turnaround time and cost.
An example of a traditional mask 10 is shown in FIG. 1. The mask 10 is formed on a transparent substrate 12 such as quartz. Opaque material 14 such as Chromium (Cr) is deposited on the substrate 12 to form a pattern of alternating opaque areas 14 and clear areas 13. As shown in FIG. 1, the width W of the clear areas 13 is equal to the width W of the opaque areas 14. The minimum width W of the alternating areas 13, 14 is referred to as the line width or feature size.
As the semiconductor manufacturers attempt to decrease the size of integrated circuits, the line width naturally continues to shrink. One known problem with conventional masks is that diffraction causes the light pattern transmitted throughout the mask to "blur." This problem is particularly acute as the line width reaches submicron levels. This problem has led to the use of phase shifting masks.
Phase shifting masks shift (usually by 180 degrees) the phase of light transmitted through every other clear area. The phase shift is accomplished by providing phase shift areas, or wells, in every other clear area. The wells may be provided in different ways. For example, FIG. 2 illustrates a phase shifting mask 100 including a substrate 12 into which phase shifting wells 16 have been etched. The phase shifting wells 16 correspond to every other clear area 13. Thus, the light transmitted through the mask 100 on one side of an opaque area 14 is phase-shifted with respect to the light transmitted through the mask 100 on the other side of the opaque area 14. As used herein, the term "non-phase shift area" refers to a clear area that is not in a phase shift well, while the terms "phase shift area" and "phase shift well" refer to a clear area that is recessed with respect to non-phase shift areas. FIG. 3 illustrates a second type of phase shifting mask 200. Rather than etching phase shifting wells into the substrate 12, a transparent material 18 is provided on top of the substrate 12 in all areas except every other clear area 13. Thus, each opaque area 14 and every other clear area 17 are on the transparent material 18, while the remaining clear areas 13 are formed in phase shifting wells 16 in which no transparent material 18 is present. The discussion herein will focus on the type of phase shifting mask 100 illustrated in FIG. 2, although those of skill in the art will readily recognize that the discussion and invention are equally applicable to the type of phase shifting mask 200 illustrated in FIG. 3.
FIG. 4 illustrates bump and divot defects in a phase shifting mask 100. It should be noted that the defects that are being discussed herein (sometimes referred to as clear area defects) are different from defects in the opaque areas 14 such as the clear and opaque defects discussed in my co-pending application entitled "Method for Removing the Carbon Halo Caused by FIB Clear Defect Repair of a Photomask," Ser. No. 09/190,057, filed Nov. 12, 1998. FIG. 4 illustrates a phase shift well 16 with a bump defect 20, as well as a phase shift well 16 with a divot defect 22. FIG. 4 also illustrates a divot defect 22 in non-phase shift clear area 17. Bump defects do not occur in the non-phase shift area in the type of phase shifting mask 100 illustrated in FIGS. 2 and 4, but may occur in the type of phase shifting mask 200 illustrated in FIG. 3 depending upon the flatness of the coated transparent material areas 18.
Methods of correcting bump and divot defects in phase shifting masks are taught in U.S. Pat. No. 5,382,484 (the "'484 patent"). The methods taught therein, however, suffer from drawbacks which make their use relatively difficult and expensive in practice. First, the methods taught by the '484 patent require the identification of the type of defect (i.e. bump or divot) in addition to identifying the existence of the defect. Although the presence of a clear area defect can often be detected with an optical mask inspection tool, in practice the identification of the type of defect often requires the use of a device such as a SEM or an AFM. Second, the methods taught by the '484 patent require tools such as FIBs (focused ion beam) and laser tools. Use of both of these tools adds to the cost and complexity of repairing defects in masks, as well as adding the risk that the mask substrate or opaque areas will be damaged by the FIB or laser tool used in the repair process.
What is needed is a method of repairing bump and divot defects in phase shifting masks that can be performed without requiring the identification of the type of defect and without requiring the use of expensive and complex tools which may damage the mask.